The present invention relates to a catheter and technique for application of laser energy to a patient. More specifically, it relates to a catheter and technique for endovascular myocardial revascularization. In other words, it involves use of a device within a patient's heart to create channels other than the coronary arteries, which channels can supply oxygenated blood and remove waste products from the myocardial tissue.
Myocardial infarctions (heart attacks) are the major pathological killer in America, resulting in the deaths of more than 500,000 persons each year. The flow of blood is compromised as atherosclerotic plaques develop within the coronary arteries, constricting their diameter, and quite often death ensues due to the complications inherent with the ischemia or infarction of the myocardium. Despite important advances which have been made in preventing and treating atherosclerotic cardiovascular disease, coronary artery impairments continue to constitute a major health problem. The most pervasive method of treating acute coronary artery occlusions is bypass surgery; however, opening of the thoracic cavity entails great pain and risks for the patient.
An alternative to bypass surgery is percutaneous transluminal angioplasty (using balloons, lasers, or a combination), a technique for delivering an object capable of removing lesions from affected arteries via a catheter. Serious possible side effects include acute closure or rupture of the blood vessel. Furthermore, there are significant numbers of afflicted individuals who are not candidates for either of these therapeutic procedures for the treatment of myocardial ischemia. These include patients with severe small-vessel diseases, diabetes, and those who could not withstand the trauma of cardiopulmonary bypass. More advanced techniques would be welcome for handling these cases, especially a means for the direct recanalization of the afflicted myocardium.
In the procedure known as balloon angioplasty, a thin catheter which contains at its distal end a tiny deflated balloon is threaded through an artery to the location of blockage. When in place in the region of occlusion, the balloon is inflated, and the obstructing plaque in the blood vessel is compressed against the arterial wall. This procedure is much less costly and traumatic to the patient than coronary bypass surgery. Many patients have been subjected to this therapy, and many of them have been relieved of symptoms. Unfortunately, it is often found that restenosis occurs over time because, basically, nothing has been removed from the artery. Although some patients receive a long term cure, others find that blockage reoccurs in about six months time.
There have been a number of reports of attempts to vaporize atherosclerotic plaque using laser radiation delivered to the site of the occlusion through flexible optical fibers carried within a catheter. While the long-wavelength Er and CO.sub.2 lasers may be well suited to the task, fibers presently available do not concurrently meet transmission specifications, required cost levels, and flexibility requirements. Roughly 30% of laser procedures presently result in either perforation or dissection of the vessel, and restenosis due to wall damage remains a significant cause of failure during long-term follow up.
As an alternative procedure, direct myocardial revascularization can provide a supply of channels other than the coronary arteries to supply oxygenated blood and remove waste products from the myocardial tissue. Because the heart has the ability to use conduits and communicating channels to perfuse itself, several different approaches have been taken for exploring the possibility of direct revascularization of the ischemic myocardium. Techniques of revascularization are disclosed in U.S. Pat. No. 5,061,265 issued to Dr. George S. Abela and hereby incorporated by reference. One revascularization technique attempted to introduce collateral circulation using myopexy (roughening the myocardial surface to enhance capillarization) and omentopexy (sewing the omentum over the heart to provide a new blood supply). Another approach involved implanting the left internal mammary artery directly into heart muscle so that blood flowing through side branches of this artery could perfuse the muscle. The blood was distributed by sinusoids and communicating networks within the myocardium. Similar techniques have involved polyethylene tubes, endocardial incisions, the creation of channels with various types of needles, and needle acupuncture.
The needle acupuncture approach rests on the critical observation that in the hearts of vipers and reptiles, myocardial perfusion occurs via communicating channels between the left ventricle and the coronary arterial tree. Thus there exist central channels in the reptilian heart that radiate from the ventricular cavity and perfuse the thickness of the myocardial wall during systole. Reptiles do not have to rely on the functioning of coronary arteries in the same manner as mammals must. This finding is the central thesis underlying attempts to duplicate the reptilian vascular pattern in the mammalian heart. However, it has been shown that the channels formed by acupuncture all close within two or three months due to fibrosis and scaring. Therefore such mechanical techniques have been largely abandoned in favor of the use of lasers to effect transmyocardial canalization. The most important advantage of channels created by a laser is that there is no mechanical injury to the heart muscle, because the channels are created by vaporization. Reports indicate that in laser canalization, fibrosis and scaring are limited, and the laser-produced channels remain patent for more than two years.
The unique properties possessed by certain types of lasers that can be applied to myocardial revascularization include removal of tissue by vaporization; high absorption of the light wave by biological tissue; rapid vaporization with little thermal damage to surrounding tissue; and precise selection of the tissue to be removed. Studies based on using the CO.sub.2 laser to create channels relied on penetration of the wall of the heart by focussing the laser on the epicardium and ablating tissue until the endocardium was penetrated. Studies on mongrel dogs indicated that a 400 W CO.sub.2 laser was able to penetrate through the wall of the heart in microseconds, and that channel size could be controlled by the external optical system. When the canine hearts were later examined, it was found that epicardial sites of the laser channeling were marked by dots of fibrous tissue, which apparently is able to heal the outer surface punctures and prevent chronic bleeding. These plugs only penetrated about one millimeter into the channel, so the revascularization was successful. Early results on human subjects also appear to be promising. However, conventional surgical procedures were required, including opening of the chest cavity and cardiopulmonary bypass.
There are several problems associated with passing laser energy through an optical fiber to reach the heart or any inside part of a patient. If laser energy passes from the proximal end (i.e., input end, outside of patient) to the distal end of an optical fiber catheter within a patient, the laser energy looses some of its coherence. It will spread as a conical beam upon exiting the distal end or tip of the optical fiber. Since its power density distribution drops off with the square of the radius, any tissue offset sufficiently from the tip will be subject to a beam of lower power density. This may result in undesirable burning, charring, and/or heating of the tissue, instead of the ablation of the tissue desired under various circumstances. This may prevent or minimize the effectiveness of a given medical procedure, especially where the spreading or diffusion of the laser beam causes it to strike healthy tissue adjacent to the desired target. In cardiovascular applications, the burning, charring, and heating may inhibit desired growth of spongy tissue at the treated sites.
Another problem with applying lasers internally to a patient is that the wavelengths of light which will pass through a regular optical fiber catheter do not include the wavelength which the body tissues will most readily absorb. Using other wavelengths of light increases the risk that the laser beam energy will puncture the wall of a vessel or other body tissue before the laser energy is completely absorbed. In other words and for example, a laser beam which must pass through 2 millimeters of tissue before complete absorption is more likely to puncture than one which is completely absorbed in 1 millimeter. The relatively slow drop off of power in the beam direction of such less than ideal wavelengths for absorption makes it difficult to precisely control the depth of the ablation or the resulting hole. One can decrease the danger of puncture by reducing the beam power, but this may increase the problem of low power density causing undesirable burning, charring, and/or heating of the tissue, instead of the ablation of the tissue. (Notwithstanding some of these problems, the incorporated by reference '743 disclosed in both its background and its various designs the use of lasers for recanalization, among other uses.)
Recent developments of zirconium fluoride and sapphire fibers have been described as paving the way for possible endoscopic applications of short penetration depth lasers, but use of non-standard optical fibers introduces possible problems. Moreover, such suggestions would still require passage of the treatment beam (i.e., laser beam which actually strikes the patient's tissues) through fibers which cause loss of coherence and beam spreading.
The direct application (i.e., without passage through an optical fiber) of a laser beam to a patient's tissue such as the heart for revascularization avoids the spreading of the beam and associated problems, but requires major surgery such as, in the case of revascularization, opening of the chest cavity. This increases the risk to the patient. In the case of revascularization by transmyocardial canalization, the laser beam is directly applied to the outside of a heart and is used to create a channel extending into the interior of the heart (i.e., a through channel). This causes bleeding and later clotting may or may not significantly block the channel which was formed. Although some early results are promising, there is still a danger that the clotting will negate the benefits of the created channels.